1. Field of the Disclosure
This disclosure relates generally to cyclic alkene compositions that exhibit stability to air and/or heat. More particularly, this disclosure is directed to cyclic alkene derivatives stabilized with one or more antioxidant compounds (e.g., substituted phenols) to reduce or eliminate polymer formation upon exposure of a cyclic alkene composition to oxygen, heat or the two in combination, and methods for use of such compositions to form dielectric films.
2. Background of the Disclosure
The semiconductor industry requires numerous types of thin and thick films to prepare semiconductor devices, many of which are based on silicon. The elemental composition of these films is typically some combination of silicon and carbon with various combinations of oxygen, hydrogen, and fluorine. In U.S. Pat. No. 6,914,335, Andideh et al. teach how layers can differ and can be used for different purposes, while in U.S. Pat. No. 6,846,515, Vrtis et al. teach ranges of silicon, oxygen, carbon and hydrogen for dielectric films preferred by the semiconductor industry. A frequently used process is chemical vapor deposition, and there are numerous variations of this process.
In a typical chemical vapor deposition process, a silicon containing compound is introduced into a deposition chamber containing a substrate to be coated. The silicon containing compound is then chemically or physically altered (i.e., reacted with another component, or subjected to application of an energy source such as radiation, heat (thermal CVD), or plasma (PECVD), etc.) to deposit a film on the substrate. Deposited films containing only silicon and oxygen (i.e., silicon oxide) have a dielectric constant of approximately 4 in the absence of pores, while films that also contain carbon (i.e., carbon doped silicon oxide) and/or pores often have dielectric constants lower than 4. Films with a dielectric constant below about 2.7 are preferred for newer semiconductor devices. In U.S. Pat. No. 6,583,048, Vincent et al. provide examples of chemical vapor deposition techniques, dielectric constants, and examples of films that are desirable in the semiconductor industry.
The properties of a layer deposited on a substrate, such as dielectric constant, film hardness and refractive index, are influenced by changing the composition of the chemistry that is fed into the film deposition tool and the process employed. The film properties can be tuned by changing the identity of the silicon containing compound by using a different flow gas, by using one or more different reactive gases, or by using post-deposition anneal techniques. Another means to affect the layer properties is to use a combination of silicon containing compounds or to combine a silicon containing compound(s) with one or more additive compounds. These techniques can be employed to alter the chemical composition of the film to adjust the film to the desired properties. U.S. Pat. Nos. 6,633,076, 6,217,658, 6,159,871, 6,479,110 and 6,756,323, herein incorporated by reference, give examples of how film properties are affected by changing deposition parameters or component mixtures.
An alternative use for the additive compound is to provide compounds whose fragments or atoms are only temporarily resident in the film. The film can be post-treated to drive the fragments or atoms out of the film using heat, radiation or a combination of heat or radiation and reactive gases, such as oxygen, to create pores in the resulting film. This approach affects the properties (e.g. dielectric constant) of the deposited film. The compounds employed in this manner are described as porogens.
Typical porogens used in this type of approach are predominately composed of carbon and hydrogen. Examples of some of the classes of cyclic alkene compounds of interest as porogens are described in U.S. Pat. Nos. 6,846,515 and 6,756,323.
High volume semiconductor manufacturing places stringent demands on the equipment and on the purity and stability of the chemistries that flow through the equipment. A chemical that is sent through chemical lines and a vaporizer means is expected to transport and vaporize cleanly and leave behind little or no residue during extended use. The longer a piece of equipment can operate between scheduled or unscheduled maintenance periods (e.g., to clean out or replace chemical lines or a vaporizer means that is fouled or clogged with polymeric or other residue), the more productive the tool is, making it more cost-effective. A deposition tool that must be shut down often for cleaning and maintenance is not as appealing to semiconductor manufacturing customers. Thus, continuous, long term operation of equipment is desirable. Vaporizer means can include several types of vaporization apparatuses, including, but not limited to, heated vaporizers (see, e.g., U.S. Pat. Nos. 6,604,492, 5,882,416, 5,835,678 and references therein), bubbler ampoules (see, e.g., U.S. Pat. Nos. 4,979,545, 5,279,338, 5,551,309, 5,607,002, 5,992,830 and references therein), flash evaporators (see, e.g., U.S. Pat. No. 5,536,323 and references therein) and misting apparatuses (see, e.g., U.S. Pat. Nos. 5,451,260, 5,372,754, 6,383,555 and references therein).
1,3,5,7-Tetramethylcyclotetrasiloxane (TMCTS) is a representative silicon containing compound which can be employed to produce low k dielectric films and is an example of the difficulty in maintaining stability. Initial work to establish reliable manufacturing processes was hampered by the product gelling at different points in the deposition process, including the chemical lines, vapor delivery lines, and within the deposition chamber. This indicated that the stability of pure TMCTS was not sufficient, and a variety of additives were studied by Teff et al. in U.S. Pat. Nos. 7,129,311 and 7,531,590, which are incorporated herein by reference. It was found that antioxidants were highly effective to stabilize TMCTS against exposure to air, specifically oxygen, for extended periods of time at ambient or elevated temperatures. When antioxidant-stabilized TMCTS is used now in semiconductor manufacturing, processes are more stable, and gel formation in a deposition tool is reduced significantly.
Norbornadiene (NBDE) is an example of a cyclic diene of interest for use as a porogen primarily due to the bond strain in its structure and its tendency to undergo thermal reactions to form volatile materials when heated (see, e.g., U.S. Pat. Nos. 6,846,515, 6,479,110, 6,437,443, and 6,312,793). NBDE and similar cyclic alkene derivatives can react with oxygen to either polymerize or oxidize, forming higher molecular weight, lower volatility materials which may or may not be soluble in the cyclic alkene monomer. This reaction can cause significant degradation of the cyclic alkene over time, even after brief air exposure at room temperature.
NBDE forms highly soluble, low volatility solid products in the presence of adventitious air, in the presence of heat, or when the two are combined. While evidence of thermal degradation has been observed in samples heated at 120° C. for 24 hours, oxidative degradation has been observed in samples kept at room temperature or samples that were heated to 80° C. or more. These are very important factors to consider, since it only takes trace oxygen (low ppb level) to form enough residue (low ppm level) to become problematic during semiconductor processing. The combined difficulty of completely eliminating oxygen from a product with the need to use heat to evaporate the product during semiconductor processing makes it nearly impossible to avoid forming low volatility residue without an effective stabilizer. This can result in accumulation of the solid product in a vaporizer means as the volatile NBDE is evaporated away. If the surface area of the vaporizer means is small, it is possible that small amounts of residue (e.g., at a milligrams level) can hinder the evaporation of NBDE, eventually causing the vaporizer means to clog with the low volatility solids. If a bubbler ampoule is employed as the vaporizer means, oxidation products could initiate a polymerization process, causing the entire contents of the bubbler to polymerize and block the flow gas inlet line. This is especially true with bubblers that are constantly heated to assist the vaporization process. The only remedy is to disassemble and clean or replace the affected vaporizer means, which is very costly and time consuming. Safety issues are also a concern if pressurized chemical lines and valves become blocked with the low volatility solid.
While NBDE is being used in semiconductor manufacturing, equipment may go idle from time to time for various reasons (e.g., power fluctuation, holiday shutdown, etc). During this idle time, a portion of product (usually <1 mL) may be kept in a heated zone at temperatures up to 85° C. for several hours or days. During this time, the product can form soluble, nonvolatile residue that will accumulate in the vaporizer when the equipment is restarted. Therefore, an effective NBDE containing composition must be thermally stable for periods of hours or days.
The standard manufacturing process for a product such as stabilized NBDE involves a significant number of chemical handling steps. Since each handling step (and subsequent storage period) is not completely free of air, product will almost always be exposed to trace amounts of oxygen during its lifetime. As mentioned previously, it takes only a trace amount of oxygen to give an unacceptable level of residue. Therefore, an effective NBDE product must also be stabilized against oxygen-induced degradation for a period of time (potentially one or more years) in order for it to have an acceptable shelf life.
The semiconductor industry requires stable, predictable and reliable products, and even a relatively low level of this decomposition is unacceptable for high volume semiconductor manufacturing. Therefore, it is necessary to find a means to stabilize NBDE to ensure that the product does not easily decompose during transport from the chemical supplier to the end-use process, even after exposure to various conditions. However, chemistry of the cyclic alkene compounds differs considerably from the chemistry of the silicon containing compounds typically employed, so it is not obvious that the same compounds that stabilize the silicon containing compounds will stabilize the cyclic alkene compounds.
TMCTS is believed to ring open and polymerize in the presence of oxygen. Further, TMCTS has Si—H bonds that are reactive with molecular oxygen (see, e.g., U.S. Patent Application No. 20040127070 and U.S. Pat. No. 6,858,697). By contrast, NBDE will slowly oligomerize in the presence of air, but it will not gel and the ring structure remains intact during the oligomerization process. Where TMCTS can completely polymerize as a gel inside a chemical line upon exposure to air, NBDE instead forms a highly soluble, medium to high molecular weight and low volatility oligomer that is not apparent upon visual inspection, or easily detectable by gas chromatography (GC). Instead, the resulting oligomers are detected when the volatile NBDE is evaporated away to leave behind the low volatility oligomers.
NBDE and similar materials are sometimes stabilized with antioxidants, such as 2,6-di-tert-butyl-4-methoxyphenol (BHA) or 2,6-di-tert-butyl-4-methylphenol (BHT), (see Clariant LSM 171779 Norbornadiene Specification Sheet and Aldrich Catalog Number B3,380-3). The known antioxidants for these materials have very high boiling points, (b.p. of BHT is 265° C.), and may have atoms not desired to be in incorporated into the deposited film (e.g. sulfur, nitrogen). These antioxidants are commonly added at concentrations of 0.02 to 0.25 wt % (200 to 2,500 ppm), but additives can exceed this amount when the manufacturer wants to increase shelf life. Chemical manufacturers prefer to use BHT due to its low cost and availability. However, the concentrations of these additives are higher than desired for semiconductor purposes. In U.S. Patent Application Nos. 20070057234 and 20070057235 (both herein incorporated by reference), Teff et al. demonstrated results using the antioxidant 4-methoxyphenol at lower concentrations.